JP7639805B2 - Grain-oriented electrical steel sheet - Google Patents

Description

The present invention relates to grain-oriented electrical steel sheets.

Grain-oriented electrical steel sheets are used, for example, as materials for the iron cores of transformers. In transformers, it is necessary to suppress energy loss. Of this, energy loss is affected by the iron loss of grain-oriented electrical steel sheets.

Here, the iron loss of grain-oriented electrical steel sheet mainly consists of hysteresis loss and eddy current loss. Techniques developed to improve hysteresis loss include highly orienting the (110)[001] orientation, known as the GOSS orientation, in the rolling direction of the steel sheet, and reducing impurities in the steel sheet. Techniques developed to improve eddy current loss include increasing the electrical resistance of the steel sheet by adding Si, and applying coating tension in the rolling direction of the steel sheet. However, these techniques have limitations in the manufacture of grain-oriented electrical steel sheet.

Therefore, to further reduce the iron loss of grain-oriented electrical steel sheets, magnetic domain refinement technology has been developed. Magnetic domain refinement technology is a technology that reduces the iron loss, especially the eddy current loss, of grain-oriented electrical steel sheets in the following way. That is, after finish annealing (hereinafter also referred to as final annealing) or after baking of the insulating coating, non-uniformity of magnetic flux is introduced into the steel sheet by physical means such as forming grooves or introducing localized distortion. This refines the width of the 180° magnetic domains (main magnetic domains) formed along the rolling direction of the steel sheet, reducing the iron loss, especially the eddy current loss, of the grain-oriented electrical steel sheet.

For example, Patent Document 1 states:
"A wound core for a transformer with low iron loss, characterized in that the wound core is made by winding and overlapping grain-oriented electromagnetic steel sheets on one surface of which linear grooves are formed with a groove width of 300 μm or less, a groove depth of 100 μm or less, a groove center line spacing of 1 mm or more in the rolling direction, and an angle of 30° or more with the rolling direction, the linear grooves facing the inward winding side, and the radius of curvature of the innermost bent portion is 30 mm or less."
has been disclosed.
The technique of forming grooves on the surface of a steel sheet to subdivide magnetic domains as described in Patent Document 1 is also called a heat-resistant magnetic domain subdivision technique because the magnetic domain subdivision effect does not disappear even when stress relief annealing is performed. Here, stress relief annealing is a heat treatment for releasing strain that is inevitably introduced into the grain-oriented magnetic steel sheet, for example, by bending the grain-oriented magnetic steel sheet to make it into a wound core. Note that this strain is different from the strain introduced by the magnetic domain subdivision treatment and has an adverse effect on iron loss.

In addition, Patent Document 2 states:
"A method for manufacturing low-iron-loss grain-oriented silicon steel sheet, characterized in that a highly convergent sheet-shaped plasma flame is radiated onto the surface of grain-oriented silicon steel sheet that has been subjected to final annealing to subdivide magnetic domains."
has been disclosed.
The technique of refining magnetic domains by introducing thermal strain into a steel sheet as described in Patent Document 2 is also called a non-heat-resistant magnetic domain refinement technique because the magnetic domain refinement effect disappears due to strain relief annealing.

Furthermore, as a technique for forming grooves on the surface of the above-mentioned steel sheet to subdivide the magnetic domains (heat-resistant magnetic domain subdivision technique), Patent Document 3 proposes an electrolytic etching method in which grooves are formed on the surface of the steel sheet by electrolytic etching. Patent Document 4 proposes a laser method in which a high-power laser is used to locally melt and evaporate the steel sheet. Patent Document 5 proposes a gear press method in which a roll on a gear is pressed against the steel sheet to create an indentation.

Special Publication No. 6-022179 Japanese Unexamined Patent Publication No. 7-192891 JP 2012-77380 A JP 2003-129135 A Japanese Patent Application Laid-Open No. 62-086121

In recent years, from the perspective of energy conservation and environmental regulations, there has been a demand to reduce not only the energy loss of transformers, but also the noise they make when in operation. The noise made by transformers when in operation is influenced by the magnetostrictive properties of grain-oriented electrical steel sheets. For this reason, it has become extremely important to develop grain-oriented electrical steel sheets that combine low iron loss with good magnetostrictive properties.

Of the above magnetic domain refinement technologies, the non-heat-resistant magnetic domain refinement technology described in Patent Document 2 can significantly reduce eddy current loss by introducing localized strain into the steel sheet. However, on the other hand, non-heat-resistant magnetic domain refinement leads to increased hysteresis loss and deterioration of magnetostriction characteristics due to the introduction of such strain. Also, as mentioned above, in grain-oriented electrical steel sheets that have been subjected to non-heat-resistant magnetic domain refinement technology, the magnetic domain refinement effect disappears due to strain relief annealing, limiting its applications.

On the other hand, in heat-resistant magnetic domain refinement technology such as those in Patent Documents 1, 3 to 5, the effect is greater the larger the area of the groove portion in the rolling direction cross section of the steel sheet. However, forming the groove deep in the thickness direction of the steel sheet leads to deterioration of the magnetic properties of the steel sheet, such as a decrease in magnetic permeability. In addition, manufacturing disadvantages such as breakage of the steel sheet also occur. As such, because heat-resistant magnetic domain refinement technology limits the depth of the grooves in the steel sheet, there is currently a demand for further reduction in iron loss while maintaining high magnetic permeability.

The present invention was developed in consideration of the above-mentioned current situation, and aims to provide a grain-oriented electrical steel sheet that has low core loss, high magnetic permeability, and good magnetostriction characteristics.

As a result of extensive research into achieving the above object, the inventors have come to the following findings.
(a) In the magnetic domain refinement technique, regions with different magnetic permeability are formed in the rolling direction, and magnetic poles are formed at the interfaces between the regions. In order to reduce the increased magnetostatic energy caused by these magnetic poles, the width of the main magnetic domain of the steel sheet is refined.
(b) In the heat-resistant magnetic domain refinement technology, the gaps in the grooves are used as regions of different magnetic permeability. These gaps do not change even when stress relief annealing is performed, so the magnetic domain refinement effect is maintained. However, the gaps in the grooves reduce the magnetic permeability of the entire steel sheet.
(c) On the other hand, in the non-heat-resistant magnetic domain refinement technique, thermal strain is introduced into the steel sheet by irradiation with an energy beam. Due to the residual stress generated by this thermal strain, new magnetic domains (closure domains) having magnetization components in the [010] or [100] crystal orientation (when the [001] crystal orientation is the rolling direction) are formed. Here, the [010] or [100] crystal orientation corresponds to the intermediate direction between the sheet width direction and the sheet thickness direction (direction at 45° from the sheet width direction to the sheet thickness direction), respectively. Since the magnetization directions of the main magnetic domain and the closure domain are different, a change in magnetic permeability occurs in the rolling direction, and magnetic domain refinement occurs. This thermal strain is eliminated by strain relief annealing, and the magnetic domain refinement effect also disappears by strain relief annealing. However, since both the main magnetic domain and the closure domain are formed inside the grain-oriented electrical steel sheet, the magnetic permeability of the steel sheet as a whole hardly changes.

The inventors therefore thought that if it were possible to form closure domains in a heat-resistant magnetic domain refinement technology that would maintain residual stress inside the steel sheet even after stress relief annealing, it might be possible to obtain an extremely high iron loss reduction effect and suppress a decrease in magnetic permeability, even without forming grooves deep in the thickness direction of the steel sheet.

Based on the above idea, the inventors conducted further research and discovered that by locally melting and solidifying the steel plate using an energy beam to form a linear molten solidified portion that crosses the rolling direction on one surface of the steel plate, residual stress can be maintained inside the steel plate even after stress relief annealing.

式（２）～（４）中、σ_ＲＤ、σ_ＴＤおよびσ_ＮＤはそれぞれ、圧延方向、板幅方向および板厚方向の残留応力である。ε_ＲＤ、ε_ＴＤおよびε_ＮＤはそれぞれ、圧延方向、板幅方向および板厚方向の歪みである。 The inventors also formed linear molten solidified portions crossing the rolling direction on one surface of a grain-oriented electrical steel sheet under various conditions.
As a result, the inventors discovered that by setting the deepest position of the region where ΔE (KJ/m ³ ) is negative in the distribution of anisotropic energy from the surface of the molten solidified portion in the thickness direction in the rolling direction cross section of the grain-oriented electrical steel sheet to a distance of 4 μm or more from the surface of the molten solidified portion, a high iron loss improvement effect can be obtained while ensuring high magnetic permeability and good magnetostriction characteristics of the steel sheet as a whole even during strain relief annealing.
Here, ΔE is defined by the following formula (1).
ΔE=E _sub -E _RD ...(1)
In formula (1), E _RD (KJ/m ³ ) and E _sub (KJ/m ³ ) are the anisotropic energy in the rolling direction, and the anisotropic energy in the crystal [010] orientation or [100] orientation when the crystal [001] orientation is the rolling direction, respectively, and are calculated by the following formulas (2) to (4).

In the formulas (2) to (4), σ _RD , σ _TD and σ _ND are the residual stresses in the rolling direction, the width direction and the thickness direction, respectively. ε _RD , ε _TD and ε _ND are the strains in the rolling direction, the width direction and the thickness direction, respectively.

Furthermore, the inventors have conducted further studies and found that even better effects can be obtained by satisfying the following points.
In the distribution of anisotropic energy from the surface of the molten solidified portion in the rolling direction cross section of the grain-oriented electrical steel sheet to the sheet thickness direction, the deepest position of the region where ΔE (KJ/m ³ ) is −2 KJ/m ³ or less is set to a distance of 4 μm to 40 μm from the surface of the molten solidified portion.
A groove defined by a molten and solidified portion is formed on one surface of the grain-oriented electrical steel sheet.
In the depth profile of the groove (cross-sectional shape of the groove) as shown in FIG. 1, the depth d of the deepest point of the groove is set to less than 8.0 μm.
In the depth profile of the groove, at least two minimum values are provided, and the ratio of the depth d of the deepest point of the groove to the width W of the groove, d/W×100(%), is set to be 5% or more and less than 20%.
The distance between the molten and solidified portions in the rolling direction is 1.0 mm or more and less than 5.0 mm.
The present invention was completed based on the above findings and through further investigation.

式（２）～（４）中、σ_ＲＤ、σ_ＴＤおよびσ_ＮＤはそれぞれ、圧延方向、板幅方向および板厚方向の残留応力である。ε_ＲＤ、ε_ＴＤおよびε_ＮＤはそれぞれ、圧延方向、板幅方向および板厚方向の歪みである。 That is, the gist and configuration of the present invention are as follows.
1. Grain-oriented electrical steel sheet,
The grain-oriented electrical steel sheet has, on one surface thereof, periodically linear molten and solidified portions that cross the rolling direction,
In the distribution of anisotropic energy from the surface of the molten solidified portion in the sheet thickness direction in a cross section in the rolling direction of the grain-oriented electrical steel sheet, the deepest position of a region where ΔE (KJ/m ³ ) is negative is 4 μm or more away from the surface of the molten solidified portion.
Here, ΔE is defined by the following formula (1).
ΔE=E _sub -E _RD ...(1)
In formula (1), E _RD (KJ/m ³ ) and E _sub (KJ/m ³ ) are the anisotropic energy in the rolling direction, and the anisotropic energy in the crystal [010] orientation or [100] orientation when the crystal [001] orientation is the rolling direction, respectively, and are calculated by the following formulas (2) to (4).

In the formulas (2) to (4), σ _RD , σ _TD and σ _ND are the residual stresses in the rolling direction, the width direction and the thickness direction, respectively. ε _RD , ε _TD and ε _ND are the strains in the rolling direction, the width direction and the thickness direction, respectively.

2. The grain-oriented electrical steel sheet according to 1, wherein in the distribution of the anisotropic energy, the deepest position of the region where the ΔE (KJ/m ³ ) is −2 KJ/m ³ or less is 4 μm to 40 μm away from the surface of the molten solidified portion.

3. The grain-oriented electrical steel sheet according to 1 or 2, having a groove portion defined by the molten solidified portion.

4. The grain-oriented electrical steel sheet according to 3, in which the depth d of the deepest point of the groove in the depth profile of the groove is less than 8.0 μm.

5. There are two or more minimum values in the depth profile of the groove portion,
5. The grain-oriented electrical steel sheet according to claim 3 or 4, wherein a ratio of a depth d of the deepest point of the groove to a width W of the groove, d/W×100(%), is 5% or more and less than 20%.

6. The grain-oriented electrical steel sheet according to any one of 1 to 5, wherein the spacing of the molten solidified portions in the rolling direction is 1.0 mm or more and less than 5.0 mm.

According to the present invention, it is possible to obtain a grain-oriented electrical steel sheet having low iron loss, high magnetic permeability, and good magnetostriction characteristics. Furthermore, the grain-oriented electrical steel sheet of the present invention has low iron loss even after strain relief annealing, and has high magnetic permeability and good magnetostriction characteristics, so it can be used extremely advantageously as an iron core material for transformers, particularly wound core transformers.

FIG. 4 is a schematic diagram showing an example of a depth profile of a groove (cross-sectional shape of a groove). FIG. 13 is a schematic diagram showing an example of anisotropic energy distribution. FIG. 11 is a diagram showing the relationship between the deepest position where ΔE<0 and iron loss W _17/50 . FIG. 4 is an enlarged view of a portion of FIG. 3 . FIG. 11 is a diagram showing the relationship between the deepest position where ΔE≦−2 and iron loss W _17/50 . FIG. 6 is an enlarged view of a portion of FIG. 5 . FIG. 13 is a diagram showing the relationship between the deepest position where ΔE<0 is satisfied and the magnetostrictive harmonic MHL _15/50 . FIG. 13 is a diagram showing the relationship between the deepest position of ΔE≦−2 and magnetostrictive harmonic MHL _15/50 . FIG. ₁₃ is a diagram showing the relationship between the deepest position of ΔE<0 and ΔB8. FIG. 13 is a diagram showing the relationship between the deepest position of ΔE≦−2 and ΔB ₈ . FIG. 11 is a diagram showing the relationship between the depth d of the deepest point of the groove and iron loss W _17/50 . FIG. 11 is a diagram showing the relationship between the depth d of the deepest point of the groove and the magnetostrictive harmonic MHL _15/50 . FIG. ₁₃ is a diagram showing the relationship between the depth d of the deepest point of the groove portion and ΔB8. FIG. 1 is a diagram showing the relationship between d/W×100 and iron loss W _17/50 . FIG. 13 is a diagram showing the relationship between d/W×100 and magnetostrictive harmonic MHL _15/50 . FIG. ₁₃ is a diagram showing the relationship between d/W×100 and ΔB8. FIG. 1 is a diagram showing the relationship between the interval of the molten solidified portion and iron loss W _17/50 . FIG. 1 is a diagram showing the relationship between the interval between molten and solidified portions and magnetostrictive harmonics MHL _15/50 . FIG. ₁₃ is a diagram showing the relationship between the spacing of the molten solidified portion and ΔB8. FIG. 4 is a schematic diagram showing a method for measuring a width W of a groove portion.

First, the experimental results that led to the completion of the present invention will be described.
(Experiment 1)
A plurality of samples of the same shape were cut out from grain-oriented electrical steel sheets (steel strips) manufactured by a general manufacturing process, and the magnetic properties, specifically, B ₈ and W _17/50 were measured by the Epstein method described in JIS C2550. The measured values of B ₈ and W _17/50 were 1.9350 T and 0.880 W/kg, respectively. Here, B ₈ means the magnetic flux density when magnetized in the rolling direction with a magnetizing force of 800 A/m. W _17/50 means the iron loss value when an alternating magnetization of 1.7 T and 50 Hz is applied in the rolling direction. Next, a laser was irradiated to each sample under various conditions in the range of laser output density: 0.1 to 1.0 (J/mm ² ) so as to cross the rolling direction, and the sample was locally melted and solidified to form a linear melted and solidified portion on the surface of the sample. The power density of the laser is expressed as P/(v·Φ) using the scanning speed (hereinafter also referred to as the deflection speed) v, the spot diameter in the direction perpendicular to the scanning direction Φ, and the laser power P. A single-mode fiber laser was used as the laser source, and no assist gas was used. The laser irradiation was performed at intervals of 4.0 mm in the rolling direction.

After the laser irradiation, each sample was subjected to stress relief annealing under nitrogen atmosphere at 800°C for 3 hours. Next, _B8 and W17 _/50 were measured for each sample by the Epstein method described in JIS C2550. In addition, _ΔB8 (=[B8 measured on the sample after laser irradiation (hereinafter also referred to as the post-irradiation sample)] - [ _B8 measured on the sample before laser irradiation (hereinafter also referred to as the pre-irradiation sample)]) was calculated for _{each sample} as an index of magnetic permeability change.

Next, the magnetostrictive vibration waveform of each post-irradiation sample was measured using a laser Doppler magnetostrictive vibrometer when it was magnetized with a 1.5 T, 50 Hz sinusoidal AC current. The measured magnetostrictive vibration waveform was then Fourier-decomposed into vibration acceleration components with frequencies of every 100 Hz. Next, the values of each frequency component, which were corrected for hearing on the A scale, were integrated from 0 to 1000 Hz, and the integrated value was taken as the magnetostrictive harmonic MHL _15/50 , which is an index of magnetostrictive properties.

Next, each post-irradiation sample was embedded in a mold and mirror-polished, after which the strain distribution in the rolling direction cross section of the sample was determined by the EBSD Wilkinson method. From the obtained strain distribution, ΔE was calculated using the above formulas (1) to (4), and the distribution of anisotropic energy from the surface of the molten solidified portion to the plate thickness direction in the rolling direction cross section of the sample was determined. For reference, FIG. 2 shows a schematic diagram of an example of the distribution of the anisotropic energy. Note that the grain-oriented electrical steel sheet having the anisotropic energy distribution illustrated in FIG. 2 does not have a groove portion defined by the molten solidified portion, that is, the surface of the grain-oriented electrical steel sheet is flat.

From the anisotropic energy distribution of each sample thus obtained, the deepest position of the region where ΔE (KJ/m ³ ) is negative (hereinafter also referred to as the deepest position where ΔE < 0) and the deepest position of the region where ΔE (KJ/m ³ ) is -2 KJ/m ³ or less (hereinafter also referred to as the deepest position where ΔE ≦ -2) were obtained.

3 to 10 show the above measurement results plotted against the deepest position where ΔE<0 and the deepest position where ΔE≦−2.
Here, FIG. 3 shows the relationship between the deepest position where ΔE<0 and the core loss W _17/50 (of the irradiated sample), and FIG. 4 is an enlarged view of a part of FIG.
FIG. 5 shows the relationship between the deepest position where ΔE≦−2 and the core loss W _17/50 (of the irradiated sample), and FIG. 6 is an enlarged view of a part of FIG.
FIG. 7 shows the relationship between the deepest position where ΔE<0 and the magnetostrictive harmonic MHL _15/50 .
FIG. 8 shows the relationship between the deepest position where ΔE≦−2 and the magnetostrictive harmonic MHL _15/50 .
FIG. 9 shows the relationship between the deepest position where ΔE<0 and _ΔB8 .
FIG. 10 shows the relationship between the deepest position where ΔE≦−2 and ΔB ₈ .

From Figures 3 and 4, it can be seen that a high iron loss improvement effect is obtained when the deepest position where ΔE<0 is 4 μm or more. It can also be seen that a particularly high iron loss improvement effect is obtained when the deepest position where ΔE<0 is 6 μm or more. The inventors believe that the reason for this is that even after stress relief annealing, stress remains inside the steel sheet due to the presence of the molten solidified part, and closure domains are generated. On the other hand, when the deepest position where ΔE<0 is less than 4 μm (including when the deepest position where ΔE<0 is 0 μm, i.e., when ΔE is all positive), a sufficient iron loss improvement effect is not obtained. The inventors believe that the reason for this is that closure domains are not generated sufficiently.

Figures 5 and 6 show that a particularly high iron loss improvement effect can be obtained when the deepest position of ΔE≦-2 is 4 μm or more. The inventors believe that the reason for this is that the magnetic anisotropy of the closure domains increases, allowing the closure domains to exist up to high magnetic fields during excitation, resulting in a higher magnetic domain refinement effect.

From Fig. 7, it can be seen that the magnetostrictive harmonic MHL _15/50 tends to increase as the deepest position of ΔE<0 becomes deeper. The inventors believe that the reason for this is that the generation and disappearance of closure domains occurs in the AC magnetization process, making the magnetostrictive vibration more complex. In particular, when the deepest position of ΔE<0 becomes 120 μm, the magnetostrictive harmonic MHL _15/50 increases rapidly. Therefore, the deepest position of ΔE<0 is preferably 100 μm or less, more preferably 50 μm or less.

8, it can be seen that, as the deepest position of ΔE≦-2 becomes deeper, the magnetostrictive harmonic MHL _15/50 tends to increase, similar to the deepest position of ΔE<0. Therefore, the deepest position of ΔE≦-2 is preferably 80 μm or less, more preferably 40 μm or less.

9 and 10, even if the deepest position of ΔE<0 and the deepest position of ΔE≦-2 are deep, _ΔB8 is less than −0.0010 T, and almost no deterioration in magnetic permeability is observed. The inventors believe that the reason for this is that the melted solidified portion grows epitaxially from the surrounding GOSS orientation grains, so that the melted solidified portion also maintains the GOSS orientation.

From the above results, in order to achieve low iron loss, low magnetostriction, and high magnetic permeability even after stress relief annealing, it is necessary to set the deepest position where ΔE<0 to 4 μm or more. In addition, it is preferable to set the deepest position where ΔE<0 to 6 μm or more. Furthermore, it is preferable to set the deepest position where ΔE<0 to 100 μm or less, and more preferably to set it to 50 μm or less.
Furthermore, in order to obtain a higher effect, it is preferable that the deepest position where ΔE≦−2 is 4 μm or more and 80 μm or less, and it is more preferable that it is 4 μm or more and 40 μm or less.

(Experiment 2)
A number of samples of the same shape were cut out from grain-oriented electrical steel sheets (steel strips) manufactured by a general manufacturing process, and B ₈ and W _17/50 were measured by the Epstein method described in JIS C2550. The measured values of B ₈ and W _17/50 were 1.9350 T and 0.880 W/kg, respectively. Next, an electron beam was irradiated across the rolling direction for each sample, and the sample was locally melted and solidified to form a linear melted solidified portion and a groove defined by the melted solidified portion on the surface of the sample. The output density of the electron beam was adjusted so that the deepest position of ΔE≦-2 was 15 μm. The output density of the electron beam is expressed as P/(v·Φ) using the deflection speed v, the spot diameter Φ in the direction perpendicular to the scanning direction, and the electron beam output P. A thermionic gun with a LaB6 chip as the cathode was used as the electron gun. Electron beam irradiation was performed at intervals of 4.0 mm in the rolling direction. In addition, the shape of the electron beam spot was made elliptical with its major axis in the scanning direction by using a focusing coil, and irradiation was performed with the ellipticity changed variously within the range of 1.1 to 12 for each sample.

After the electron beam irradiation, each sample was subjected to stress relief annealing under a nitrogen atmosphere at 800°C for 3 hours. Next, _B8 and W17 _/50 were measured for each sample by the Epstein method described in JIS C2550. In addition, _ΔB8 (=[B8 measured on the sample after laser irradiation (hereinafter also referred to as the post-irradiation sample)]-[ _B8 measured on the sample before laser irradiation (hereinafter also referred to as the pre-irradiation sample)]) was calculated for _each sample as an index of magnetic permeability change. In addition, in the same manner as in Experiment 1, magnetostrictive harmonic MHL15 _/50, which is an index of magnetostrictive properties, was obtained for each post-irradiation sample.

Next, for each post-irradiation sample, a depth profile of the groove was created from its surface using a laser microscope, and the depth d of the deepest point of the groove (the distance in the sheet thickness direction between the deepest point of the groove and the surface of the grain-oriented electrical steel sheet) was measured.

11 to 13 show the above measurement results plotted against the depth d of the deepest point of the groove portion.
Here, FIG. 11 shows the relationship between the depth d of the deepest point of the groove and the core loss W _17/50 (of the irradiated sample).
FIG. 12 shows the relationship between the depth d of the deepest point of the groove and the magnetostrictive harmonic MHL _15/50 .
FIG. 13 shows the relationship between the depth d of the deepest point of the groove and _ΔB8 .

As can be seen from FIG. 11, the greater the depth d of the deepest point of the groove, i.e., the deeper the groove, the greater the tendency for iron loss to improve. In particular, it can be seen that a high iron loss improvement effect can be obtained when the depth d of the deepest point of the groove is 1.0 μm or more. The inventors believe that the reason for this is that the free magnetic pole formed on the wall surface surrounding the deepest point of the groove promotes magnetic domain refinement.

It can be seen from FIG. 12 that the magnetostrictive harmonic MHL _15/50 is hardly affected by the depth d of the deepest point of the groove.

13, _ΔB8 , which is an index of change in magnetic permeability, tends to increase as the depth d of the deepest point of the groove increases, that is, as the groove becomes deeper. In particular, when the depth d of the deepest point of the groove exceeds 8.0 μm, _ΔB8 tends to increase significantly. The inventors believe that the reason for this is that the groove becomes deeper, increasing the volume of the groove, which is a void, and thus deteriorating the magnetic permeability.

Based on the above results, in order to achieve low iron loss, low magnetostriction, and high magnetic permeability even after stress relief annealing, it is preferable that the depth d of the deepest point of the groove is less than 8.0 μm. The depth d of the deepest point of the groove is more preferably less than 7.0 μm. Furthermore, the depth d of the deepest point of the groove is more preferably 1.0 μm or more.

(Experiment 3)
A number of samples of the same shape were cut out from grain-oriented electrical steel sheets (steel strips) manufactured by a general manufacturing process, and B ₈ and W _17/50 were measured by the Epstein method described in JIS C2550. The measured values of B ₈ and W _17/50 were 1.9350 T and 0.880 W/kg, respectively. Next, a laser was irradiated across the rolling direction for each sample, and the sample was locally melted and solidified to form a linear melted solidified portion and a groove defined by the melted solidified portion on the surface of the sample. The laser power density was adjusted so that the deepest position of ΔE≦-2 was 15 μm. The laser power density is expressed as P/(v·Φ) using the deflection speed v, the spot diameter Φ in the direction perpendicular to the scanning direction, and the laser output P. A single-mode fiber laser was used as the laser source, and no assist gas was used. The laser irradiation was performed at intervals of 4.0 mm in the rolling direction. In addition, a cylindrical lens was used to shape the laser spot into an ellipse having a major axis in the scanning direction, and irradiation was performed with the ellipticity changed variously within the range of 1.1 to 12 for each sample.

After the laser irradiation, each sample was subjected to stress relief annealing under a nitrogen atmosphere at 800°C for 3 hours. Next, _B8 and W17 _/50 were measured for each sample by the Epstein method described in JIS C2550. In addition, _ΔB8 (=[B8 measured on the sample after laser irradiation (hereinafter also referred to as the post-irradiation sample)]-[ _B8 measured on the sample before laser irradiation (hereinafter also referred to as the pre-irradiation sample)]) was calculated for _each sample as an index of magnetic permeability change. In addition, in the same manner as in Experiment 1, magnetostrictive harmonic MHL15 _/50, which is an index of magnetostrictive properties, was obtained for each post-irradiation sample.

Next, for each post-irradiation sample, a depth profile of the groove was created from its surface using a laser microscope, and the groove width W, the depth d of the deepest point of the groove, and the number of minimum values N were measured. The number of maximum values is N-1. If there are no grooves, that is, if the surface of the steel plate is flat, the number of minimum values and maximum values in the groove will both be 0. The method for measuring the depth profile of the groove will be described later.

14 to 16 show the above measurement results, plotted against d/W×100(%) for each number N of minimum values.
Here, FIG. 14 shows the relationship between d/W×100 and the core loss W _17/50 (of the irradiated sample).
FIG. 15 shows the relationship between d/W×100 and magnetostrictive harmonic MHL _15/50 .
FIG. 16 shows the relationship between d/W×100 and _ΔB8 .

As can be seen from Figure 14, when d/W x 100 is 5% or more, there is a tendency for a high iron loss improvement effect to be obtained. The inventors believe that the reason for this is that the slope of the groove defined by the molten solidified portion approaches vertical, or the groove is formed deep, increasing the number of magnetic poles generated on the wall surface. It can also be seen that the higher the number of minimum values N, especially when it is 2 or more, the higher the iron loss improvement effect can be obtained. The inventors believe that the reason for this is that the shape of the molten solidified portion becomes more complex, causing greater stress to remain inside the steel sheet, making it possible to maintain the closure domains up to a higher magnetic field.

15, when d/W×100 is 20% or more, the magnetostriction harmonic MHL _15/50 tends to increase. In particular, as the number N of minimum values increases, this tendency becomes more pronounced. The inventors believe that the reason for this is that as the shape of the molten solidified portion becomes more complex, greater stress remains inside the steel sheet, and more closure domains are generated.

From FIG. 16, it can be seen that _ΔB8 , which is an index of change in magnetic permeability, is hardly affected by d/W×100 and the number N of minimum values.

From the above results, in order to achieve low core loss, low magnetostriction, and high magnetic permeability even after stress relief annealing, it is preferable that d/W x 100 is 5% or more and less than 20%, and the number N of minimum values is 2 or more. d/W x 100 is more preferably 7% or more. d/W x 100 is more preferably 18% or less.

(Experiment 4)
A number of samples of the same shape were cut out from grain-oriented electrical steel sheets (steel strips) manufactured by a general manufacturing process, and B ₈ and W _17/50 were measured by the Epstein method described in JIS C2550. The measured values of B ₈ and W _17/50 were 1.9350 T and 0.880 W/kg, respectively. Next, a laser was irradiated across the rolling direction for each sample, and the sample was locally melted and solidified to form a linear melted solidified portion and a groove portion defined by the melted solidified portion on the surface of the sample. The laser power density was adjusted so that the deepest position of ΔE≦-2 was 20 μm, the number N of minimum values in the depth profile of the groove was two, and d/W×100 was 10%. The laser power density is expressed as P/(v·Φ) using the deflection speed v, the spot diameter Φ in the direction perpendicular to the scan, and the laser power P. A single-mode fiber laser was used as the laser source, and no assist gas was used. The rolling direction interval of the laser irradiation was varied in the range of 0.5 to 7.0 mm for each sample. The shape of the laser spot was set to a line segment shape in which the scanning direction was the stretching direction using a diffractive optical element, and the ratio of the line segment length Ll to the spot width Lw in the rolling direction, Ll/Lw, was varied in the range of 1.1 to 6.

After the laser irradiation, each sample was subjected to stress relief annealing under a nitrogen atmosphere at 800°C for 3 hours. Next, _B8 and W17 _/50 were measured for each sample by the Epstein method described in JIS C2550. In addition, _ΔB8 (=[B8 measured on the sample after laser irradiation (hereinafter also referred to as the post-irradiation sample)]-[ _B8 measured on the sample before laser irradiation (hereinafter also referred to as the pre-irradiation sample)]) was calculated for _each sample as an index of magnetic permeability change. In addition, in the same manner as in Experiment 1, magnetostrictive harmonic MHL15 _/50, which is an index of magnetostrictive properties, was obtained for each post-irradiation sample.

17 to 19 show the above measurement results plotted against the interval of laser irradiation in the rolling direction, that is, the interval of the molten solidified portion in the rolling direction (hereinafter also referred to as the interval of the molten solidified portion).
Here, FIG. 17 shows the relationship between the interval between the molten and solidified portions and the iron loss W _17/50 (of the irradiated sample).
FIG. 18 shows the relationship between the interval between the molten and solidified portions and the magnetostrictive harmonic MHL _15/50 .
FIG. 19 shows the relationship between the interval between the molten and solidified portions and _ΔB8 .

As can be seen from Figure 17, the smaller the spacing between the molten and solidified parts, the greater the iron loss improvement effect tends to be. The inventors believe that the reason for this is that as the spacing between the molten and solidified parts becomes smaller, the area of the molten and solidified parts, and therefore the interface area between the closure domain and the main domain, increases, and the total amount of magnetic poles increases.

18, the smaller the interval between the molten and solidified parts, the more the magnetostrictive harmonics MHL _15/50 tend to increase. The inventors believe that the reason for this is that the magnetostrictive vibration becomes more complicated as the total number of closure domains increases.

From FIG. 19, it can be seen that _ΔB8 , which is an index of change in magnetic permeability, is hardly affected by the interval between the molten and solidified portions.

Based on the above results, in order to achieve low iron loss, low magnetostriction, and high magnetic permeability even after stress relief annealing, it is preferable to set the spacing between the molten solidified portions to 1.0 mm or more and less than 5.0 mm. The spacing between the molten solidified portions is more preferably 2.0 mm or more. The spacing between the molten solidified portions is more preferably 4.0 mm or less.

式（２）～（４）中、σ_ＲＤ、σ_ＴＤおよびσ_ＮＤはそれぞれ、圧延方向、板幅方向および板厚方向の残留応力である。ε_ＲＤ、ε_ＴＤおよびε_ＮＤはそれぞれ、圧延方向、板幅方向および板厚方向の歪みである。
なお、方向性電磁鋼板の圧延方向断面における異方性エネルギーの分布などの測定、および、後述する溝部の形状（深度プロファイル）に係る測定は、板幅中心位置で行えばよく、特に断りがなければ、板幅中心位置で測定したものである。 Next, a grain-oriented electrical steel sheet according to an embodiment of the present invention, which has been completed based on the above experimental results, will be described.
The grain-oriented electrical steel sheet according to one embodiment of the present invention has linear molten solidified portions periodically crossing the rolling direction on one surface thereof,
In the distribution of anisotropic energy from the surface of the molten solidified portion in the sheet thickness direction in the rolling direction cross section of the grain-oriented electrical steel sheet, the deepest position of the region where ΔE (KJ/m ³ ) is negative is a distance of 4 μm or more from the surface of the molten solidified portion.
Here, ΔE is defined by the following formula (1).
ΔE=E _sub -E _RD ...(1)
In formula (1), E _RD (KJ/m ³ ) and E _sub (KJ/m ³ ) are the anisotropic energy in the rolling direction, and the anisotropic energy in the crystal [010] orientation or [100] orientation when the crystal [001] orientation is the rolling direction, respectively, and are calculated by the following formulas (2) to (4).

In the formulas (2) to (4), σ _RD , σ _TD and σ _ND are the residual stresses in the rolling direction, the width direction and the thickness direction, respectively. ε _RD , ε _TD and ε _ND are the strains in the rolling direction, the width direction and the thickness direction, respectively.
Measurements of the distribution of anisotropic energy in the cross section in the rolling direction of the grain-oriented electrical steel sheet and measurements of the shape (depth profile) of the groove portion described later may be performed at the center position of the sheet width, and unless otherwise specified, the measurements are performed at the center position of the sheet width.

First, it is important that the grain-oriented electrical steel sheet according to one embodiment of the present invention has a linear molten solidified portion on one surface that crosses the rolling direction. Here, the molten solidified portion is a region in which the base material of the grain-oriented electrical steel sheet is melted by heat, cooled, and solidified. In addition, a heat-affected zone is usually formed adjacent to the molten solidified portion. The molten solidified portion is defined as follows. That is, the grain-oriented electrical steel sheet is cut so that the rolling direction cross section of the grain-oriented electrical steel sheet becomes the cut surface. Next, distortion-free polishing is performed on the cut surface. Next, the crystal orientation difference of the polished surface is analyzed by EBSD (Electron Backscatter Diffractoin), and the KAM (Karnel Average Misorientation) value is measured. The area with a KAM value of 3.0° or more is the molten solidified area, the area with residual elastic strain in the strain distribution analyzed by the EBSD Wilkinson method described below and with a KAM value of less than 3.0° is the heat-affected area, and the other areas (areas with no residual elastic strain and with a KAM value of less than 3.0°) are the base material area.

In the non-heat-resistant magnetic domain refinement technique described in Patent Document 2, the thermal strain introduced into the steel sheet by the non-heat-resistant magnetic domain refinement and the residual stress inside the steel sheet due to the thermal strain are released by strain relief annealing. In contrast, in the grain-oriented electrical steel sheet according to one embodiment of the present invention, the residual stress inside the steel sheet caused by the molten solidified portion is maintained even if strain relief annealing is performed.
The inventors consider the reason for this to be as follows.
That is, when a steel plate is locally heated and melted, it is rapidly cooled from the molten metal state and solidified. It is presumed that this process causes solidification to be completed before the distortion caused by the phase transformation of the molten part is fully eliminated. In particular, it is believed that such residual distortion becomes large because the molten solidified part has a complex shape with multiple minimum values in the depth profile of the groove part defined by the molten solidified part.

In a grain-oriented electrical steel sheet according to one embodiment of the present invention, the deepest position where ΔE<0 is a distance (in the sheet thickness direction) from the surface of the molten solidified portion of the sheet must be 4 μm or more; in other words, the region where ΔE is negative must extend to a depth of 4 μm or more from the surface of the molten solidified portion.

Deepest position of ΔE<0: 4 μm or more from the surface of the molten solidified portion According to the above-mentioned experimental results, the deepest position of ΔE<0 must be 4 μm or more from the surface of the molten solidified portion. The preferred range of the deepest position of ΔE<0 is as shown in the above-mentioned experimental results.

The preferred range for the deepest position where ΔE≦-2 is also as shown in the experimental results described above. The deepest position where ΔE≦-2 is most preferably a distance (in the plate thickness direction) from the surface of the molten solidified portion of 4 μm to 40 μm.

Here, the deepest position where ΔE<0 and the deepest position where ΔE≦-2 are determined from the distribution of anisotropic energy in the thickness direction from the surface of the molten solidified part in the rolling direction cross section of the grain-oriented electrical steel sheet. The anisotropic energy at each position in the distribution of anisotropic energy is basically calculated at 1 μm pitch. However, for areas where a certain trend can be grasped, the anisotropic energy may be calculated at a larger pitch, for example, 5 to 10 μm pitch.

For example, in the distribution of anisotropic energy, if ΔE from the surface of the molten solidified portion to a position of 10 μm deep is negative and ΔE at a position of 11 μm deep is positive, the deepest position of ΔE < 0 is 10 μm. Similarly, if ΔE from the surface of the molten solidified portion to a position of 10 μm deep is -2 KJ / m ³ or less and ΔE at a position of 11 μm deep is -1 KJ / m ³ , the deepest position of ΔE ≦ -2 is 10 μm.

In addition, the distribution of anisotropic energy from the surface of the molten solidified portion in the rolling direction cross section of the grain-oriented electrical steel sheet to the sheet thickness direction (ΔE at various depth positions) is created using the above formulas (1) to (4) from the strain distribution from the surface of the molten solidified portion in the rolling direction cross section of the grain-oriented electrical steel sheet to the sheet thickness direction (ε _RD , ε _TD and ε _ND at various depth positions). In addition, the strain distribution from the surface of the molten solidified portion in the rolling direction cross section of the grain-oriented electrical steel sheet to the sheet thickness direction (ε _RD , ε _TD and ε _ND at various depth positions) is obtained by the EBSD Wilkinson method. In the EBSD Wilkinson method, an electron beam is irradiated to the rolling direction cross section of the grain-oriented electrical steel sheet, and a Kikuchi pattern is obtained for each measurement point. Then, using the no-strain point as a reference point, the amount of strain is calculated from the deformation amount of the Kikuchi pattern at each measurement point using analysis software such as CrossCourt. In addition, when a groove portion defined by the molten solidified portion exists, the strain distribution from the deepest point of the groove portion in the rolling direction cross section to the plate thickness direction is measured, and the distribution of anisotropic energy is created. When no groove portion exists (the steel plate surface is flat), the strain distribution from the width center of the surface of the molten solidified portion in the rolling direction cross section to the plate thickness direction is measured, and the distribution of anisotropic energy is created.

In addition, it is preferable that the grain-oriented electrical steel sheet according to one embodiment of the present invention has a groove portion defined by the above-mentioned molten solidified portion. Even if there are two minimum values in the depth profile of the groove portion as shown in Figure 1, if the groove is defined by one molten solidified portion (the wall surface, bottom (valley portion), and peak portion of the groove are composed of one molten solidified portion), it is considered to be one groove portion.

Here, it is preferable that the shape of the groove is such that the depth d of the deepest point of the groove in the depth profile of the groove is less than 8 μm. The reason for this is as shown in the experimental results described above. In addition, a more preferable range for the depth d of the deepest point of the groove is also as shown in the experimental results described above.

It is also preferable that the depth profile of the groove has two or more minimum values, and that the ratio of the depth d of the deepest point of the groove to the width W of the groove, d/W×100(%), is 5% or more and less than 20%. The reason for this is as shown in the experimental results described above. The number N of minimum values and the more preferable range of d/W×100(%) are also as shown in the experimental results described above.

Here, the depth profile of the groove is created as follows.
That is, the surface of the grain-oriented electrical steel sheet is observed using a laser microscope, and the depth of the groove is continuously measured at 1 μm pitch along the direction perpendicular to the extension direction of the molten solidified part. Then, in the sheet thickness direction, the side where the groove (molten solidified part) exists is set as +, the opposite side is set as -, and the steel sheet surface level is set as 0. The measured depth of the groove is smoothed (smoothed) by a three-point moving average method and then plotted to create a depth profile of the groove. Then, from the created depth profile of the groove, the points at which the minimum and maximum values are taken are determined, and the number of points at which the minimum and maximum values are taken is obtained. In addition, the distance from the surface of the steel sheet to the point at which the minimum value is taken in the depth profile of the groove is set as the depth d of the deepest point of the groove.

In addition, the width W of the groove is measured as the distance between the ends of the groove in the direction perpendicular to the extension of the molten solidified portion at the level of the steel plate surface, as shown in Figure 20. Note that Figure 20 is a schematic diagram showing an example of the cross-sectional shape of the molten solidified portion (groove), and the direction perpendicular to the paper surface is the extension direction of the molten solidified portion (groove).

The maximum value of the depth profile of the groove is preferably such that the molten solidified portion does not protrude from the surface of the steel sheet, i.e., is less than 0 μm. The reason for this is that if the molten solidified portion protrudes outward in the thickness direction from the surface of the steel sheet, when the grain-oriented electromagnetic steel sheets are stacked as a transformer core, the space factor decreases and distortion is introduced into the steel sheet on the upper surface during stacking, resulting in deterioration of iron loss and noise. The maximum value of the depth profile of the groove is preferably greater than -d (μm), and more preferably greater than -d/2 (μm).

Furthermore, the spacing between the molten solidified portions is preferably 1.0 mm or more and less than 5.0 mm. The reason for this is as shown in the experimental results described above. Furthermore, the more preferable range for the spacing between the molten solidified portions is also as shown in the experimental results described above.

Here, the spacing between the molten solidified portions is the center-to-center distance between the molten solidified portions in the rolling direction.

In addition, the more the extension direction of the molten solidified portion is tilted from the plate width direction (the direction perpendicular to the rolling direction), the fewer magnetic poles are generated at the interface between the closure domain and the main magnetic domain, and the effect of magnetic domain refinement tends to deteriorate. Therefore, it is preferable that the angle between the extension direction of the molten solidified portion and the plate width direction is within ±30°.

The composition of the grain-oriented electrical steel sheet according to an embodiment of the present invention is not particularly limited. For example, a preferred composition is as follows:
C: 50 mass ppm or less,
Si: 2.0 to 8.0% by mass,
Mn: 0.005 to 1.0% by mass,
Al: 0.065% by mass or less,
N: 0.0120% by mass or less,
S: 0.030% by mass or less and Se: 0.030% by mass or less;
Optionally,
Contains one or more selected from Ni: 1.50% by mass or less, Sn: 1.50% by mass or less, Sb: 1.50% by mass or less, Cu: 3.0% by mass or less, P: 0.50% by mass or less, Mo: 0.10% by mass or less, and Cr: 1.50% by mass or less;
The balance is Fe and unavoidable impurities. The preferred content of each element will be described below.

C: 50 ppm by mass or less C can be contained in the slab to improve the hot-rolled sheet structure. However, in order to avoid the occurrence of magnetic aging during the manufacturing process, the C content contained in the slab is preferably reduced to 50 ppm by mass or less by decarburization annealing, and the C content of the final product, the grain-oriented electrical steel sheet, is also equivalent to this. Therefore, the C content is preferably 50 ppm by mass or less. The lower limit of the C content is not particularly limited and may be 0 ppm by mass.

Si: 2.0 to 8.0% by mass
Silicon is an element that is effective in increasing the electrical resistance of steel and improving iron loss. Therefore, the silicon content is preferably 2.0 mass% or more. However, if the silicon content exceeds 8.0 mass%, there is a risk of deterioration in workability and threadability, and a decrease in magnetic flux density. Therefore, the silicon content is preferably in the range of 2.0 to 8.0 mass%.

Mn: 0.005 to 1.0% by mass
Mn is an element useful for improving hot workability. Therefore, the Mn content is preferably 0.005 mass% or more. On the other hand, if the Mn content exceeds 1.0 mass%, there is a risk of a decrease in magnetic flux density. Therefore, the Mn content is preferably in the range of 0.005 to 1.0 mass%.

Al: 0.065% by mass or less, N: 0.0120% by mass or less, S: 0.030% by mass or less, and Se: 0.030% by mass or less As described later, the composition of the grain-oriented electrical steel sheet slab may be a composition that uses an inhibitor or a composition that does not use an inhibitor, but in either composition, the grain-oriented electrical steel sheet, which is the final product, is preferably Al: 0.065% by mass or less, N: 0.0120% by mass or less, S: 0.030% by mass or less, and Se: 0.030% by mass or less. Even in the case of a composition that uses an inhibitor, if purification is performed in the final annealing, the inhibitor components are removed. Therefore, the contents of these elements are more preferably Al: 0.010% by mass or less, N: 0.0050% by mass or less, S: 0.0050% by mass or less, and Se: 0.0050% by mass or less, respectively. The lower limit of the content of these elements is not particularly limited, and may be 0 mass %.

As optional added components, at least one selected from Ni: 1.50 mass% or less, Sn: 1.50 mass% or less, Sb: 1.50 mass% or less, Cu: 3.0 mass% or less, P: 0.50 mass% or less, Mo: 0.10 mass% or less, and Cr: 1.50 mass% or less Ni is an element effective for improving the hot rolled sheet structure and improving the magnetic properties. Therefore, when Ni is contained, its content is preferably 0.03 mass% or more. However, if the Ni content exceeds 1.50 mass%, the secondary recrystallization may become unstable and the magnetic properties may deteriorate. Therefore, when Ni is contained, its content is preferably 1.50 mass% or less.

In addition, Sn, Sb, Cu, P, Mo and Cr are also elements that improve magnetic properties. Therefore, when these elements are contained, it is preferable to set Sn: 0.01 mass% or more, Sb: 0.005 mass% or more, Cu: 0.03 mass% or more, P: 0.03 mass% or more, Mo: 0.005 mass% or more, and Cr: 0.03 mass% or more. However, if Sn: exceeds 1.50 mass%, Sb: exceeds 1.50 mass%, Cu: exceeds 3.0 mass%, P: exceeds 0.50 mass%, Mo: exceeds 0.10 mass%, and Cr: exceeds 1.50 mass%, the growth of secondary recrystallized grains is suppressed, and the magnetic properties may deteriorate. Therefore, when these elements are contained, it is preferable that Sn: 1.50 mass% or less, Sb: 1.50 mass% or less, Cu: 3.0 mass% or less, P: 0.50 mass% or less, Mo: 0.10 mass% or less, and Cr: 1.50 mass% or less.

Elements other than those mentioned above are Fe (iron) and unavoidable impurities.

The thickness of the grain-oriented electrical steel sheet according to one embodiment of the present invention is not particularly limited, but is preferably 0.15 mm or more and 0.30 mm or less.

In addition, in the grain-oriented electrical steel sheet according to one embodiment of the present invention, "having low iron loss, high magnetic permeability and good magnetostriction characteristics" means that W _17/50 , ΔB ₈ and MHL _15/50 each simultaneously satisfy the pass criteria of the examples described later.

Next, an example of a method for manufacturing a grain-oriented electrical steel sheet according to one embodiment of the present invention will be described.

The steel material (slab) of the grain-oriented electrical steel sheet is hot-rolled to form a hot-rolled steel strip. The hot-rolled steel strip is then annealed. The hot-rolled steel strip is then cold-rolled once or twice or more times to form a cold-rolled steel strip (hereinafter also referred to as steel strip) of the final thickness. The steel strip is then decarburized and coated with an annealing separator mainly composed of MgO. The steel strip is then wound into a coil and subjected to final annealing for the purpose of secondary recrystallization and the formation of a forsterite coating. After the final annealing, the steel strip is subjected to flattening annealing, and a magnesium phosphate-based tension coating is formed to form the product steel strip and grain-oriented electrical steel sheet. The conditions for hot rolling, hot-rolled steel annealing, cold rolling, decarburization annealing, final annealing, and flattening annealing are not particularly limited, and may be performed in the usual manner.

Furthermore, the composition of the steel material (slab) of the grain-oriented electrical steel sheet is not particularly limited as long as it is a composition that causes secondary recrystallization, but it is preferable that the composition be such that the above-mentioned preferred composition of the grain-oriented electrical steel sheet is obtained.
for example,
C: 0.08% by mass or less,
Si: 2.0 to 8.0% by mass,
Mn: 0.005 to 1.0% by mass
Al: 0.065% by mass or less,
N: 0.0120% by mass or less,
S: 0.030% by mass or less and Se: 0.030% by mass or less;
Optionally,
Contains one or more selected from Ni: 1.50% by mass or less, Sn: 1.50% by mass or less, Sb: 1.50% by mass or less, Cu: 3.0% by mass or less, P: 0.50% by mass or less, Mo: 0.10% by mass or less, and Cr: 1.50% by mass or less;
An example of the composition is a composition in which the balance is Fe and unavoidable impurities.

In addition, in the above-mentioned component composition, when an inhibitor is used, for example, when an AlN-based inhibitor is used, Al and N may be contained, and the preferred contents of Al and N are as follows.
Al: 0.010 to 0.065% by mass
N: 0.0050 to 0.0120% by mass
When a MnS.MnSe-based inhibitor is used, Se and/or S may be contained in addition to the above-mentioned Mn, and the preferred contents of S and Se are as follows:
S: 0.005 to 0.030% by mass
Se: 0.005 to 0.030% by mass
Incidentally, both the AlN-based inhibitor and the MnS/MnSe-based inhibitor may be used in combination.

When no inhibitor is used, the preferred contents of Al, N, S and Se are as follows:
Al: 0.010% by mass or less N: 0.0050% by mass or less S: 0.0050% by mass or less Se: 0.0050% by mass or less

C can also be added to the slab to improve the hot-rolled sheet structure. However, in order to avoid the occurrence of magnetic aging during the manufacturing process, it is preferable to reduce the C content in the slab to 50 mass ppm or less by decarburization annealing. Therefore, the C content is preferably 0.08 mass% or less. Note that since secondary recrystallization occurs even without adding C, there is no particular lower limit for the C content, and it may be 0 mass%.

The reasons for limiting the elements other than those mentioned above are basically the same as the composition of the grain-oriented electrical steel sheet described above, so we will not explain them here.

Then, in any of the steps after the above-mentioned cold rolling, or between any of the steps, a process is carried out to form a molten solidified portion on the surface of the grain-oriented electrical steel sheet (or steel strip). The method for forming the molten solidified portion is not particularly limited as long as it can form the above-mentioned molten solidified portion, but examples include a method of irradiating an electron beam or a laser as an energy beam. Below, (A) the preferred conditions for irradiating an electron beam and (B) the preferred conditions for irradiating a laser are explained.

(A) Suitable conditions for irradiating electron beam In order to form a molten solidified portion on the surface of a grain-oriented electrical steel sheet, for example, it is effective to use an electron beam with high penetrability. In particular, it is necessary to control the irradiation conditions so that the temperature of the base material in the irradiated portion of the energy beam is raised above its melting temperature to melt the base material to a certain depth, while preventing the evaporation of the molten metal or suppressing it to a certain amount. Such control can be carried out, for example, by adjusting the beam output density in the range of 0.1 to 1.0 (J/mm ² ) according to the steel type and plate thickness of the grain-oriented electrical steel sheet, and adjusting each of the conditions shown below in the ranges shown below. In the case of an electron beam, the beam output density is calculated as P/(v·Φ) using the deflection speed v, the spot diameter Φ in the direction perpendicular to the scan, and the electron beam output P. In addition, the electron beam output P is calculated as the acceleration voltage x beam current. In addition, in the grain-oriented electrical steel sheet having the component composition used in the examples described later, a molten solidified portion can be formed in the range of 0.1≦P (J/mm ² ) ≦0.5, and a molten solidified portion and a groove portion defined by the molten solidified portion can be formed in the range of 0.5<P≦1.0 (J/mm ² ). However, even if the electron beam output P is within the above range, the heat input and temperature history change depending on the deflection speed v, the electron beam output P, and further the steel type and plate thickness of the grain-oriented electrical steel sheet, so that only thermal strain may be introduced and the molten solidified portion may not be formed, or the molten metal may evaporate and the molten solidified portion may not be formed. Therefore, for example, it is preferable to perform a preliminary irradiation test or the like to determine appropriate irradiation conditions in advance according to the steel type and plate thickness of the grain-oriented electrical steel sheet (the same applies to the case of laser irradiation described later).

The formation of the groove defined by the molten solidified portion and the control of the shape of the groove can be achieved by adjusting the power density as described above, and further adjusting the spot shape to have a beam energy distribution with an elliptical shape, a line segment shape, or a point sequence shape with a major axis in the scanning direction. This is expected to produce even greater effects.

Acceleration voltage: 60 kV or more and 300 kV or less Higher acceleration voltages are preferable because they increase the linearity of electrons and reduce the thermal effect on the outside of the beam irradiation area. For this reason, it is preferable that the acceleration voltage is 60 kV or more. The acceleration voltage is more preferably 90 kV or more, and even more preferably 120 kV or more. On the other hand, if the acceleration voltage is too high, it becomes difficult to shield the X-rays generated by the electron beam irradiation. Therefore, from a practical standpoint, it is preferable that the acceleration voltage is 300 kV or less. The acceleration voltage is more preferably 200 kV or less.

・Beam current: 0.5 to 40 mA
From the viewpoint of beam diameter, it is preferable that the beam current is small. This is because if the current is large, the beam diameter is likely to expand due to Coulomb repulsion. Therefore, the beam current is preferably 40 mA or less. On the other hand, if the beam current is too small, there is a risk that the energy required to form distortion will be insufficient. Therefore, the beam current is preferably 0.5 mA or more.

Degree of vacuum in the beam irradiation region The electron beam is scattered by gas molecules, which causes an increase in the beam diameter and halo diameter, a decrease in energy, etc. Therefore, the higher the degree of vacuum in the beam irradiation region, the better, and the pressure is preferably 3 Pa or less. There is no particular lower limit, but if it is lowered too much, the cost of the vacuum system, such as the vacuum pump, will increase. Therefore, in practice, the pressure is preferably 1×10 ⁻⁵ Pa or more.

Spot diameter in the orthogonal direction to scan: 300 μm or less The smaller the spot diameter, the more localized the distortion can be introduced, which is preferable. Therefore, the spot diameter of the energy beam in the orthogonal direction to scan is preferably 300 μm or less. The spot diameter of the energy beam in the orthogonal direction to scan is more preferably 280 μm or less, and even more preferably 260 μm or less. The lower limit of the spot diameter of the energy beam in the orthogonal direction to scan is not particularly limited, but the spot diameter of the energy beam in the orthogonal direction to scan is preferably, for example, 50 μm or more. The spot diameter refers to the full width at half maximum of the beam profile obtained by the slit method using a slit with a width of 30 μm.

Ratio of spot diameter in the scanning direction to spot diameter in the scanning orthogonal direction (= [spot diameter in the scanning direction (μm)] ÷ [spot diameter in the scanning orthogonal direction (μm)]): more than 1 and not more than 20. The closer the ratio of the spot diameter in the scanning direction to the spot diameter in the scanning orthogonal direction (hereinafter also referred to as the spot diameter ratio) is to 1, the more the heat input to the spot increases, and the easier it is to melt the steel sheet that serves as the base material. However, due to the concentration of the heat input area, spatter scattering is more likely to occur, and there is a risk of the space factor deteriorating when laminated as an iron core. There is also a risk of the groove portion becoming excessively deep. Therefore, the spot diameter ratio is preferably more than 1, more preferably 1.5 or more, and even more preferably 2 or more. On the other hand, the heat input area increases as the spot diameter ratio increases. Therefore, although the scattering of spatter can be suppressed, the heat input energy required to melt the steel sheet increases. In this case, there is a concern that the burden on the transport system will increase in the case of a laser, and the power supply capacity will increase in the case of an electron beam. Therefore, it is preferable that the spot diameter ratio is 20 or less.

As a method of realizing such a spot shape, for example, in the case of an electron beam, it is possible to converge the beam into spot shapes having different spot diameters by adjusting the magnetic field distribution in the focusing coil in the direction that exerts the Lorentz force in the scanning direction and the direction perpendicular to the scanning direction.
In the case of a laser, examples of the method include placing a cylindrical lens on the beam path and adjusting the spread of the beam in the scanning direction and the direction perpendicular to the scanning direction to form an elliptical shape, and placing a diffractive optical element on the beam path and focusing the beam into an elliptical shape, a line segment shape, or a dot sequence shape.

・Deflection speed: 5-400m/sec
The slower the deflection speed of the energy beam, the more the amount of heat incident on the steel sheet per unit length can be increased. However, if the deflection speed of the energy beam is made too slow, spatters may adhere to the surface of the steel sheet due to the scattering of metal vapor, and the space factor may deteriorate when laminated as an iron core. In addition, the depth of the groove may become excessive. Therefore, the deflection speed of the energy beam is preferably 5 m/sec or more. The deflection speed of the energy beam is more preferably 8 m/sec or more, and further preferably 10 m/sec or more. On the other hand, if the deflection speed of the energy beam is made too fast, a power supply capacity is required to provide the heat input required for melting the steel sheet, leading to an increase in the size of the equipment. Therefore, the deflection speed of the energy beam is preferably 400 m/sec or less.

(B) Suitable conditions for irradiating a laser In order to form a molten solidified portion on the surface of a grain-oriented electrical steel sheet, it is effective to use, for example, a laser having a wavelength of 400 nm to 1200 nm, which has a high absorption rate in metal. In particular, it is necessary to control the irradiation conditions so that the temperature of the base material in the irradiated portion of the energy beam is raised above its melting temperature to melt the base material to a certain depth, while preventing the evaporation of the molten metal or suppressing it to a certain amount. Such control can be carried out, for example, by adjusting the range of 0.1 to 1.0 (J/mm ² ) according to the steel type and plate thickness of the grain-oriented electrical steel sheet, and adjusting each of the conditions shown below within each range shown below. In the case of a laser, the output density of the beam is expressed as P/(v·Φ) using the scanning speed (deflection speed) v, the spot diameter Φ, and the laser output P.

In addition, the formation of the groove defined by the molten solidified portion and the control of the shape of the groove can be achieved by adjusting the power density as described above, and further adjusting the spot shape to have a beam energy distribution with an elliptical shape, a line segment shape, or a point sequence shape with a major axis in the scanning direction, which is expected to produce even greater effects.

Laser output: 50W or more and 5000W or less When the laser output is low, it is necessary to slow down the scanning speed in order to melt the base material, which is the steel plate. However, if the scanning speed is slowed down too much, the space factor will decrease due to the scattering of spatter on the steel plate, and the manufacturing efficiency will deteriorate. In addition, the molten metal may evaporate, and the groove may become excessively deep. On the other hand, if the laser output is increased, the steel plate will be easily melted. However, damage to the laser transport system may increase, and the manufacturing efficiency may decrease due to the increased frequency of maintenance. Therefore, the laser output is preferably 50W or more and 5000W or less.

The preferred ranges for the spot diameter in the direction perpendicular to the scanning direction, the spot diameter ratio, and the scanning speed (deflection speed) are the same as those for the preferred conditions for irradiating the electron beam (A), so a description of them will be omitted here.

The conditions for the stress relief annealing performed after bending or other processing on the grain-oriented electrical steel sheet can be exemplified as follows: nitrogen atmosphere, treatment temperature: 800°C, treatment time: 3 hours. The grain-oriented electrical steel sheet according to one embodiment of the present invention has low core loss even during stress relief annealing, and has high magnetic permeability and good magnetostriction characteristics, so it can be used extremely advantageously as an iron core material for transformers, particularly wound core transformers.

Conditions other than those mentioned above are not particularly limited and may be performed in accordance with conventional methods.

Next, the present invention will be specifically described based on examples. The following examples are intended to illustrate preferred embodiments of the present invention, and are not intended to limit the scope of the present invention. The present invention may be modified within the scope of the invention, and such modifications are also within the scope of the present invention.

Example 1
A slab of grain-oriented electrical steel sheet having the composition shown in Table 1 (the balance being Fe and unavoidable impurities) was hot-rolled to obtain a hot-rolled steel strip. The hot-rolled steel strip was then subjected to hot-rolled sheet annealing. The hot-rolled steel strip was then subjected to two cold rolling processes with intermediate annealing in between to obtain a cold-rolled steel strip (hereinafter also referred to as steel strip) having a sheet thickness of 0.23 mm. The steel strip was then subjected to decarburization annealing and coated with an annealing separator mainly composed of MgO. The steel strip was then wound into a coil and subjected to final annealing for the purpose of secondary recrystallization and formation of a forsterite coating.

Then, the steel strip was subjected to flattening annealing, and then one side of the steel strip was irradiated with an electron beam or laser to form a linear molten solidified portion. At this time, the irradiation conditions, specifically, the power density (accelerating voltage, beam current, laser output, and deflection speed) were variously changed. In addition, the spot shape was elliptical, and the spot diameter ratio was variously changed. Note that both the electron beam and the laser irradiation were performed at intervals of 4 mm in the rolling direction. In addition, a non-irradiated area was provided in a part of the steel strip.

Next, a magnesium phosphate-based tensile coating was formed on the steel strip to obtain the final product, a steel strip (grain-oriented electrical steel sheet). Note that conditions other than those specified in each of the above steps were carried out according to standard methods. In addition, the chemical composition of the steel strip that became the final product all satisfied the preferred chemical composition of the grain-oriented electrical steel sheet according to one embodiment of the present invention described above.

The steel strip thus obtained was subjected to a heat treatment simulating stress relief annealing (nitrogen atmosphere, treatment temperature: 800°C, treatment time: 3 hours). Next, a sample was cut out from the area where the linear molten solidification part was formed in the steel strip, embedded in a mold, and mirror polished. The strain distribution in the rolling direction cross section of the sample was then obtained using the EBSD Wilkinson method. From the strain distribution of each sample obtained, ΔE was calculated using the above formulas (1) to (4) to create a distribution of anisotropic energy, and the deepest position where ΔE<0 and the deepest position where ΔE≦-2 were obtained for each sample.

In addition, for each sample, _B8 and W17 _/50 were measured by the Epstein method described in JIS C2550. Furthermore, a reference sample (a sample not subjected to magnetic domain refinement treatment) was cut out from the non-irradiated region of the steel strip, and _B8 and W17 _/50 were measured by the Epstein method described in JIS C2550. Then, _ΔB8 (= [ _B8 measured on a sample cut out from a region where a linear molten solidification part was formed on the steel strip] - [ _B8 measured on a reference sample]) was calculated as an index of magnetic permeability change.

In addition, the magnetostrictive vibration waveform of each sample was measured using a laser Doppler magnetostrictive vibrometer when it was magnetized with a 1.5 T, 50 Hz sinusoidal AC current. The measured magnetostrictive vibration waveform was then Fourier-decomposed into vibration acceleration components with frequencies of every 100 Hz. Next, the A-scale auditory-corrected values of each frequency component were integrated from 0 to 1000 Hz, and the integrated value was taken as the magnetostrictive harmonic MHL _15/50 , which is an index of magnetostrictive properties.

Next, a model three-phase winding transformer (core weight 500 kg) was made using each sample, and the transformer iron loss was measured when the magnetic flux density in the core legs was 1.7 T at a frequency of 50 Hz. The no-load loss of this transformer iron loss was measured using a wattmeter. At the same time, this model transformer was excited in a soundproof room under conditions of maximum magnetic flux density Bm = 1.7 T and frequency of 50 Hz, and the noise level (dBA) of the transformer was measured using a sound level meter. Hereinafter, the noise level of the transformer is also referred to as transformer noise.

For comparison, a steel strip (No. 33) was prepared without forming a molten solidified portion (without performing magnetic domain refining treatment), and a steel strip (No. 34) was prepared in which a groove 20 μm deep was formed by laser irradiation, and W _17/50 , ΔB ₈ , MHL _15/50 , transformer iron loss, and transformer noise were measured in the same manner as above. Note that no groove was formed in any of the steel strips other than No. 34.

The above results are summarized in Table 2. The presence or absence of the molten and solidified portion was confirmed by the above-mentioned method. The evaluation criteria for W _17/50 , ΔB ₈ , MHL _15/50 , transformer iron loss and transformer noise are as follows.
・W _17/50
◎ (Pass, especially excellent): Iron loss improvement amount ΔW _17/50 is 0.08 W/Kg or more. 〇 (Pass, excellent): Iron loss improvement amount ΔW _17/50 is 0.05 W/Kg or more (excluding the case of ◎).
× (Fail): Iron loss improvement amount ΔW _17/50 is less than 0.05 W/Kg. Here, the iron loss improvement amount ΔW _17/50 is the iron loss improvement amount before and after the magnetic domain refinement treatment, and is calculated as [W _17/50 measured on a reference sample] - [W _17/50 measured on the sample].
・ΔB ₈
◎ (Pass, particularly excellent): -0.0006T or more 〇 (Pass, excellent): -0.0010T or more (except for ◎)
× (fail): −0.0010 T or less Here, ΔB ₈ is the amount of change in B ₈ before and after the magnetic domain refining process, and is calculated as [B ₈ measured on the sample]−[B ₈ measured on the reference sample].
・MHL _15/50
◎ (pass, particularly excellent): 26.0 dBA or less 〇 (pass, excellent): 27.5 dBA or less (except for ◎)
× (Fail): Over 27.5 dBA Transformer iron loss ◎ (Pass, particularly excellent): Transformer iron loss improvement of 0.08 W/Kg or more 〇 (Pass, excellent): Transformer iron loss improvement of 0.05 W/Kg or more (except for the case of ◎)
× (Fail): Improvement in transformer iron loss is less than 0.05 W/Kg. Here, the improvement in transformer iron loss is the improvement in transformer iron loss before and after magnetic domain refinement processing, and is calculated as [W17 _/50 measured on a model transformer made from a reference sample] - [W17 _/50 measured on a model transformer made from the sample in question].
・Transformer noise ◎ (pass, particularly excellent): 32.0 dBA or less 〇 (pass, excellent): 33.5 dBA or less (except for ◎)
× (fail): more than 33.5 dBA

As can be seen from Table 2, all of the examples of the invention have low iron loss, high magnetic permeability, and good magnetostriction characteristics. In addition, the transformer iron loss and transformer noise measured using a model transformer made from the samples of the examples of the invention were both low and reached the target level. Furthermore, the examples of the invention in which the deepest position for ΔE≦-2 was 4 μm to 40 μm achieved particularly excellent characteristics.

On the other hand, none of the comparative examples were able to achieve low iron loss, high magnetic permeability, and good magnetostriction characteristics.

Example 2
A slab of grain-oriented electrical steel sheet having the composition shown in Table 3 (the balance being Fe and unavoidable impurities) was hot-rolled to obtain a hot-rolled steel strip. The hot-rolled steel strip was then subjected to hot-rolled sheet annealing. The hot-rolled steel strip was then subjected to two cold rolling processes with intermediate annealing in between to obtain a cold-rolled steel strip (hereinafter also referred to as steel strip) having a sheet thickness of 0.23 mm. The steel strip was then subjected to decarburization annealing and coated with an annealing separator mainly composed of MgO. The steel strip was then wound into a coil and subjected to final annealing for the purpose of secondary recrystallization and formation of a forsterite coating.

Then, the steel strip was subjected to flattening annealing, and then a laser was irradiated onto one side of the steel strip in the rolling direction to form a linear molten solidified portion. At this time, the irradiation conditions, specifically, the power density (acceleration voltage, beam current, laser output, and deflection speed) were variously changed. In addition, the spot shape was made into a line segment shape using a diffractive optical element, and the line segment length was adjusted to variously change the spot diameter ratio. Note that in all cases, the laser irradiation was performed at intervals of 0.5 to 7.0 mm in the rolling direction. In addition, a non-irradiated area was provided in part of the steel strip.

Next, a magnesium phosphate-based tensile coating was formed on the steel strip to obtain the final product, a steel strip (grain-oriented electrical steel sheet). Note that conditions other than those specified in each of the above steps were carried out according to standard methods. In addition, the chemical composition of the steel strip that became the final product all satisfied the preferred chemical composition of the grain-oriented electrical steel sheet according to one embodiment of the present invention described above.

The steel strip thus obtained was subjected to a heat treatment simulating stress relief annealing (nitrogen atmosphere, treatment temperature: 800°C, treatment time: 3 hours). Next, in the same manner as in Example 1, anisotropic energy distribution was created, and the deepest position where ΔE<0 and the deepest position where ΔE≦-2 were determined for each sample. In addition, W _17/50 , ΔB ₈ , MHL _15/50 , transformer iron loss, and transformer noise were measured for each sample in the same manner as in Example 1.

Furthermore, using the method described above, a depth profile of the grooves of each sample was created, and the depth d (μm) of the deepest point of the grooves, the number N of minimum values, and d/W×100 (%) were determined.

Furthermore, for comparison, a steel strip (No. 58) was prepared without forming a molten solidified portion (without performing magnetic domain refinement treatment), and a steel strip (No. 59) was prepared in which a groove 20 μm deep was formed by laser irradiation, and W _17/50 , ΔB ₈ , MHL _15/50 , transformer iron loss and transformer noise were measured in the same manner as above.

The above results are summarized in Table 4. The presence or absence of the molten solidified portion was confirmed by the above-mentioned method. The evaluation criteria for W _17/50 , ΔB ₈ , MHL _15/50 , transformer iron loss, and transformer noise are the same as those in Example 1, but when the iron loss change amount ΔW _17/50 , ΔB ₈ , MHL _15/50 , transformer iron loss change amount ΔW _17/50 , and transformer noise simultaneously satisfy the following ranges, it can be said that extremely excellent characteristics have been obtained.
・ ΔW _17/50 : 0.14 W/kg or more (especially 0.15 W/kg or more)
・ΔB ₈ : -0.0002T or more (especially -0.0001T or more)
MHL _15/50 : 26.0 dBA or less (especially 25.5 dBA or less)
・Transformer iron loss ΔW _17/50 : 0.16 W/kg or more (especially 0.17 W/kg or more)
・Transformer noise: 32.0 dBA or less (especially 31.5 dBA or less)

From Table 4, all of the examples of the invention have low iron loss, high magnetic permeability, and good magnetostriction characteristics. In addition, the transformer iron loss and transformer noise measured with a model transformer made from the sample of the example of the invention were both low and reached the target level. Furthermore, in the example of the invention in which the deepest position of the region where ΔE (KJ/m ³ ) is -2 KJ/m ³ or less is 4 μm to 40 μm, there is a groove defined by the molten solidified portion, the depth d of the deepest point of the groove is less than 8.0 μm, the number N of minimum values is 2 or more, d/W×100 (%) is 5% or more and less than 20%, and the interval of the molten solidified portion is 1.0 mm or more and less than 5.0 mm, extremely excellent characteristics were obtained.

On the other hand, none of the comparative examples were able to achieve low iron loss, high magnetic permeability, and good magnetostriction characteristics.

Claims (9)

A grain-oriented electrical steel sheet,
The grain-oriented electrical steel sheet has, on one surface thereof, periodically linear molten and solidified portions that cross the rolling direction,
In the distribution of anisotropic energy from the surface of the molten solidified portion in the sheet thickness direction in a cross section in the rolling direction of the grain-oriented electrical steel sheet, the deepest position of a region where ΔE (KJ/m ³ ) is negative is a distance from the surface of the molten solidified portion of 4 μm to 150 μm .
Here, ΔE is defined by the following formula (1).
ΔE=E _sub -E _RD ...(1)
In formula (1), E _RD (KJ/m ³ ) and E _sub (KJ/m ³ ) are the anisotropic energy in the rolling direction, and the anisotropic energy in the crystal [010] orientation or [100] orientation when the crystal [001] orientation is the rolling direction, respectively, and are calculated by the following formulas (2) to (4).
In the formulas (2) to (4), σ _RD , σ _TD and σ _ND are the residual stresses in the rolling direction, the width direction and the thickness direction, respectively. ε _RD , ε _TD and ε _ND are the strains in the rolling direction, the width direction and the thickness direction, respectively. The grain-oriented electrical steel sheet according to claim 1, wherein in the distribution of the anisotropic energy, the deepest position of the region where the ΔE (KJ/m ³ ) is −2 KJ/m ³ or less is 4 μm to 40 μm away from the surface of the molten solidified portion. The grain-oriented electrical steel sheet according to claim 1, having a groove portion defined by the molten solidified portion. The grain-oriented electrical steel sheet according to claim 2, having a groove portion defined by the molten solidified portion. The grain-oriented electrical steel sheet according to claim 3, wherein in the depth profile of the groove, the depth d of the deepest point of the groove is less than 8.0 μm. The grain-oriented electrical steel sheet according to claim 4, wherein in the depth profile of the groove, the depth d of the deepest point of the groove is less than 8.0 μm. There are two or more minimum values in the depth profile of the groove,
The grain-oriented electrical steel sheet according to any one of claims 3 to 6, wherein d/W×100(%), which is a ratio of a depth d of the deepest point of the groove to a width W of the groove, is 5% or more and less than 20%. The grain-oriented electrical steel sheet according to any one of claims 1 to 6, in which the spacing between the molten solidified portions in the rolling direction is 1.0 mm or more and less than 5.0 mm. The grain-oriented electrical steel sheet according to claim 7, wherein the spacing between the molten solidified portions in the rolling direction is 1.0 mm or more and less than 5.0 mm.

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